1. Field of the Invention
The invention relates to a process for biological water treatment, in particular for denitrification of raw water to produce potable water. The invention also relates to equipment suitable for biological denitrification, in particular of potable water.
2. Description of the Related Art
The raw water used for the production of potabler water, for example ground water, is increasingly subject to environmental pollution. Above all, nitrates from fertilizers, manure or the like pass into the ground water. The potable water produced from the latter must therefore be freed from nitrate (denitrified) before consumption or at least to such an extent that it meets the statutory requirements. It can also become necessary to denitrify treated industrial water.
Multi-stage denitrification is known, in which the nitrate-containing raw water is first passed, with the addition of a reducing agent, for example ethanol, glucose or the like, into an anoxic reactor packed with carrier materials. As a result of the added reducing agent, bacteria which reduce the nitrate to molecular gaseous nitrogen are formed on the carrier material. At the same time, the carbon-containing reducing agents provide an organic carbon supply, namely as an energy carrier, for the bacteria in the reactor. The potable water pretreated to this extent then requires an aerobic, biologically active filtration. In such a filter, the excess substances added as reducing agents and biomass are to be removed from the potable water, with addition of oxygen.
A disadvantage of this known process is the discontinuous course of the denitrification. This results from the fact that the reactors must be washed at regular intervals in order to remove the excess biomass produced. Moreover, the quality of the treatment in the first process stage depends on the age of the biomass in the anoxic reactor. An expensive combination of processes for final purification of the denitrified water is therefore necessary in every case. For this reason, the known biological denitrification process requires careful process supervision and intensive servicing of the unit. With known static carrier material packing, there is a risk, in the event of uneven flow through it, of the biomass caking, as a result of which undesired nitrite can be formed.
This known process also has disadvantages in respect of equipment. In fact, in order to avoid the complete close-down during the frequently necessary washing of the reactors, several reactors should be provided, of which alternately one reactor is always in operation, while the other reactor is being back-washed.